Memory Devices
Memory devices are used in electronic systems and computers to store information in the form of binary data. These memory devices may be characterized into various types, each type having associated with it various advantages and disadvantages.
For example, random access memory ("RAM") which may be found in personal computers is volatile semiconductor memory; in other words, the stored data is lost if the power source is disconnected or removed. Dynamic RAM ("DRAM") is particularly volatile in that it must be "refreshed" (i.e. recharged) every few microseconds in order to maintain the stored data. Static RAM ("SRAM") will hold the data after one writing so long as the power source is maintained; once the power source is disconnected, however, the data is lost. Thus, in these volatile memory configurations, information is only retained so long as the power to the system is not turned off.
CD-ROM is an example of non-volatile memory. CD-ROM is large enough to contain lengthy audio and video segments; however, information can only be read from and not written to this memory. Thus, once a CD-ROM is programmed during manufacture, it cannot be reprogrammed with new information.
Other storage devices such as magnetic storage devices (i.e., floppy disks, hard disks and magnetic tape) as well as other systems, such as optical disks, are non-volatile, have extremely high capacity, and can be rewritten many times. Unfortunately, these memory devices are physically large, are shock/vibration-sensitive, require expensive mechanical drives as well as consume relatively large amounts of power. These negative aspects make these memory devices non-ideal for low power portable applications such as lap-top and palm-top computers and personal digital assistants ("PDAs").
Due to the rapidly growing numbers of compact, low-power portable computer systems in which stored information changes regularly, read/write semiconductor memories have become widespread. Furthermore, because these portable systems require data storage when the power is turned off, a non-volatile storage device is required. The simplest programmable semiconductor non-volatile memory devices in these computers are programmable read-only memory ("PROM"). The most basic PROM uses an array of fusible links; once programmed, a PROM cannot be reprogrammed. This is an example of a write-once read-many ("WORM") memory. The erasable PROM ("EPROM") is alterable, but each rewrite must be preceded by an erase step involving exposure to ultra violet light. The electrically erasable PROM ("EEPROM" or "E.sup.2 PROM") is perhaps the most ideal of conventional non-volatile semiconductor memory, as it can be written to many times. Flash memories, another type of EEPROM, have higher capacity than the low density, traditional EEPROMs but lack their endurance. One major problem with EEPROMs is that they are inherently complex. The floating gate storage elements that are used in these memory devices are difficult to manufacture and consume a relatively large amount of semiconductor real estate. Furthermore, the circuit design must withstand the high voltages necessary to program the device. This means that an EEPROM's cost per bit of memory capacity is extremely high compared with other means of data storage. Another disadvantage of EEPROMs is that although they can retain data without having the power source connected, they require relatively large amounts of power to program. This power drain can be considerable in a compact portable system powered by a battery.
Recently, a great amount of attention has been given to an alternative non-volatile memory technology based on ferroelectric materials. Unfortunately, there is still a great number of problems associated with this data storage approach that have prevented the widespread application of ferroelectric materials, the various problems including non-ideal storage characteristics and extreme difficulties in manufacturing.
Accordingly, in view of the various problems associated with conventional data storage devices described above, it is highly desirable to have a read/write memory technology and device which is inherently simple and inexpensive to produce. Furthermore, this memory technology should meet the requirements of the new generation of portable computer devices by operating under a low voltage while providing high storage density, non-volatility, and a low manufacturing cost.
Programmable Passive and Active Components
Electronic circuits may comprise literally millions of component parts. These component parts generally fall into two distinct categories, namely, passive components and active components. Passive components, such as resistors and capacitors, have electrical values associated with them which are relatively constant. On the other hand, the electrical values of active components, such as transistors, are designed to change when a voltage or current is applied to control electrodes.
Because of the extensive use of these two types of components, it is highly desirable to have a low-cost device which may perform both the functions of a passive component and an active component. For example, it would be highly desirable to have a device that acts as an active component which responds to an applied signal by altering its resistance and capacitance and yet, in an alternate embodiment, the same device acts as a passive component which can be pre-programmed (i.e., the change is "remembered" by the device after programming is complete). Such a device would be able to be implemented in many diverse applications from tuned circuits in communications equipment to volume controls in audio systems.
Optical Devices
Recently, there has been an enormous demand for various optical devices such as display devices for lap-top computers, high definition television ("HDTV"), spatial light modulators, and the like. It would be highly desirable to have a low cost, highly manufacturable device that may be utilized in such optical devices as, for example, a shutter to block the passage of light through an optical cell or as a mirror which may deflect a scanned incident beam on or off a screen or other target.
Sensors
The measurement of exposure to ultraviolet radiation and other forms of radiation is very important as radiation is believed, for example, to promote skin cancer and other damaging effects to an individual.
Accordingly, it is desirable to have a highly manufacturable device that may be used in low-cost, wavelength sensors or sensor arrays for short wavelength radiation such as ultraviolet radiation (10.sup.-7 -10.sup.-9 meters), x-rays (10.sup.-9 -10.sup.11), and gamma rays (10.sup.11 -10.sup.-14).
Conclusion
Because of the widespread use of devices such as memory devices, programmable resistor and capacitor devices, electro-optical devices, and sensors, it is very desirable to have a low cost, easy to manufacture device that may be implemented in all of these various applications, among others.